Low-cost, scalable energy storage systems are needed to improve the energy efficiency of the electrical grid (e.g., load-leveling, frequency regulation) and to facilitate the large-scale penetration of renewable energy resources (e.g., wind, solar). While alternative energy technologies exist, they cannot be directly connected to the grid because of their variable output. Electrochemical energy storage may provide the best combination of efficiency, cost, and flexibility to enable these applications. Of particular interest are redox flow batteries, which are rechargeable electrochemical energy storage devices that utilize the oxidation and reduction of two soluble electroactive species for charging (absorbing energy) and discharging (delivering energy). Unlike conventional secondary batteries, the energy-bearing species are not stored within an electrode structure but in separate liquid reservoirs and pumped to and from the power converting device when energy is being transferred. Because of this key difference, flow battery systems can be more durable than conventional battery systems as electrode reactions are not accompanied by morphological changes due to the insertion or removal of the active species and can be more scalable than conventional battery systems as the energy capacity may be easily and inexpensively modulated by varying the reservoir volume or the species concentration, without sacrificing power density. Thus, while flow batteries may not compete with compact lithium (Li)-ion batteries for portable applications (e.g., cell phones, laptops) due to lower overall energy densities, they are well-suited for large-scale stationary applications.
Since their inception in the 1960s, a large number of aqueous redox flow batteries have been developed including iron-chromium, bromine-polysulfide, vanadium-bromine, and all-vanadium systems. Several aqueous hybrid systems also have been developed, where one or both electrode reactions are a deposition/dissolution process, such as zinc-bromine and soluble lead-acid systems.
Though several of these aqueous technologies have been successfully demonstrated at the megawatt-scale, none have experienced widespread commercialization due to low energy densities, low round-trip energy efficiencies, and high costs. Indeed, all flow batteries based on aqueous electrochemical couples are limited by the electrochemical properties of water, which is only stable within a small potential window (typically 1.2-1.6 V) outside of which water electrolysis occurs. Employing non-aqueous electrolytes offers a wider window of electrochemical stability, which, in turn, enables flow batteries to operate at higher cell potentials (e.g., >2 V). If appropriate redox couples can be identified, operating at higher cell voltages leads to greater system energy (and power) densities and higher energy efficiencies. Moreover, as fewer cell units and ancillary parts would be required to achieve the same power output as an aqueous system, hardware costs would be significantly reduced and system reliability increased. In contrast to their aqueous counterparts, only a few non-aqueous flow batteries have been reported. The majority of the reported non-aqueous flow batteries are anion-exchange systems which employ single electrolytes composed of metal-centered coordination complexes. Despite their promising cell potentials, these systems have been hampered by low efficiencies and the limited solubility of coordination complexes.
All current flow battery designs have functional or cost-performance limitations that hamper large scale adoption of this technology. Thus, there is an ongoing need for new redox flow batteries. The present invention addresses this need by providing a redox flow battery that utilizes a two-electron benzodithiophene-based redox material.